Corrosion inhibitor

ABSTRACT

A corrosion inhibitor for use with Urea Ammonium Nitrate solutions is disclosed, comprising a blend of molybdate and one or more of the inorganic phosphates (including phosphates, polyphosphates, and pyrophosphates) and organic phosphates or phosphonates. Inorganic phosphates include, but are not limited to, SHMP (Sodium Hexametaphosphate) and TKPP (Tetra-Potassium Pyrophosphate). There are numerous other inorganic phosphates that will also serve as suitable secondary inhibitors. Organic phosphates or phosphonates include, but are not limited to, HEDP (1-Hydroxyethylidine-1,1-diphosphonic acid; also known as ethanol diphosphonate, acetodiphosphonic acid, or etidronic acid), ATMP or AMP (aminotri(methylenephosphonic acid)), PBTC (Phosphonobutane tricarboxylic acid), DETPMP (Diethylenetriaminepenta(methylene phosphonic acid)), and HPA (hydroxyphosphono acetic acid). There are numerous other organic phosphates and phosphonates that will also serve as suitable secondary inhibitors. The amount of molybdate can range from 1 ppm to 500 ppm by weight of fertilizer solution, with the preferred range from 10 ppm to 200 ppm. Amounts of inorganic or organic phosphate can also range from levels as low as 1 ppm up to 500 ppm, with the preferred range being from 5 ppm to 50 ppm.

FIELD OF THE INVENTION

[0001] This invention relates to corrosion inhibitors to be used withUrea Ammonium Nitrate solutions, commonly used as agriculturalfertilizer. Such solutions can be very corrosive to metals, particularlyferrous metals.

BACKGROUND OF THE INVENTION

[0002] Aqueous solutions of Urea Ammonium Nitrate (“UAN”) are commonlyused as agricultural fertilizer. However, these solutions, typically50%-80% UAN by weight (20% to 50% water), are very corrosive to metals,particularly ferrous containing metals. Manufacturers of UAN solutionshave tried a number of methods to resolve the corrosion problem.

[0003] The production of UAN solutions can be by batch or continuousprocess. Urea and Ammonium Nitrate solutions are blended with water andpH adjusted, sometimes using ammonia. During production orpost-production, efforts are made to defeat or limit the corrosiveeffects of the UAN solution. Some manufacturers use filmers, that act tocoat the metal coming into contact with the UAN solution. Filmers arequickly depleted, however, as the UAN solution comes into contact withadditional metal surfaces. Filmers can also result in more activecorrosion at localized or focused sites, where the filmer fails to fullycoat a metal surface. Filmers also are known to cause problems withfoaming and sludge formation, additional undesirable side effects of theuse of filmers.

[0004] Manufacturers have used various additives as corrosioninhibitors. For example, see U.S. Pat. No. 5,376,159 where molybdate, byitself, is used as a corrosion inhibitor. A variety of other materialshave been used as corrosion inhibitors. For example in U.S. Pat. No.3,024,100, a variety of flourosilicates in combination with molybdate isdisclosed as a corrosion inhibitor for UAN.

SUMMARY OF THE INVENTION

[0005] It has been found that low levels of molybdate in combinationwith low levels of various phosphorous containing compounds controlscorrosion in ferrous metals caused by contact with UAN solutions. Inaddition, foaming, sludge formation, and/or focused or localizedcorrosion has not been observed in connection with the use of themolybdate with certain phosphorous containing compounds. Additionally,the use of molybdate in combination with phosphorous containingcompounds prevents or reduces the formation of precipitates, includingiron and other potential precipitates. It is believed that thephosphorous containing compounds used in the instant invention act totie up or sequester iron and others materials in a manner whichmaintains the potential precipitate in solution. Precipitates can causelocalized corrosion, by creating a site which the inhibitor cannotreach, sometimes referred to as under-deposit corrosion. The avoidanceof precipitates by the use of phosphorous containing compounds preventsthe occurrence of under-deposit corrosion.

[0006] Typically an aqueous solution of alkali metal molybdate would beutilized, such as Sodium or Potassium molybdate (Na2MoO4 or K2MoO4).However, any other form of molybdate that can deliver a molybdate ioncan be used. The other additive or additives, referred to herein assecondary inhibitors, would be one or more of the inorganic phosphates(including phosphates, polyphosphates, and pyrophosphates) and organicphosphates or phosphonates. Inorganic phosphates include, but are notlimited to, SHMP (Sodium Hexametaphosphate) and TKPP (Tetra-PotassiumPyrophosphate). There are numerous other inorganic phosphates that willalso serve as suitable secondary inhibitors. Organic phosphates orphosphonates include, but are not limited to, HEDP(1-Hydroxyethylidine-1,1-diphosphonic acid; also known as ethanoldiphosphonate, acetodiphosphonic acid, or etidronic acid), ATMP or AMP(aminotri(methylenephosphonic acid)), PBTC (Phosphonobutanetricarboxylic acid), DETPMP (Diethylenetriaminepenta(methylenephosphonic acid)), and HPA (hydroxyphosphono acetic acid). There arenumerous other organic phosphates and phosphonates that will also serveas suitable secondary inhibitors.

[0007] The combination of low levels of molybdate with low levels of oneor more of these phosphorous containing compounds has resulted in lowercorrosion rates than either inhibitor used alone, even at comparabledosage rates, and has resulted in cleaner UAN solutions with little ironor other material precipitates than with either inhibitor used alone.

[0008] The dosage of molybdate and phosphorous containing compounds canvary within a wide range, limited on the low side by the amount neededto insure adequate corrosion inhibition (taking into account that UANsolution may be further diluted by the consumer prior to use), and onthe high side by the cost of the inhibitor and by the solubility of theinhibitors in the UAN solution. Amounts of molybdate can range from 1ppm to 500 ppm by weight of UAN solution, with the preferred range from10 ppm to 200 ppm. Amounts of phosphorous containing compounds can alsorange from levels as low as 1 ppm up to 500 ppm, with the preferredrange being from 5 ppm to 50 ppm.

[0009] The corrosion inhibitor combination may be added to the UANsolution in a variety of ways. The inhibitors typically would becombined, and then may be added to the UAN solution during production ofthe UAN. However, the inhibitors also may be added to the UANindependently. Alternatively, the inhibitors are often added to the UANsolution after it is produced. Vessels used for storage ortransportation of UAN solutions are often manufactured from ferrousmaterials and are subject to the corrosive effects of UAN. These vesselsmay include storage tanks, tanker trucks, barges, railroad cars, andsimilar vessels. Over time, the UAN-caused corrosion can threaten theintegrity of the vessel. In addition, the quality of the UAN solutioncan be compromised by the products of the corrosion process, includingthe precipitation of iron containing compounds.

DETAILED DESCRIPTION OF THE INVENTION

[0010] The invention disclosed concerns the prevention or limitation ofcorrosion of metals, particularly ferrous metals, when subjected tocontact with aqueous solution of UAN. Such solutions are frequently usedas agricultural fertilizers, and can be of varying strengths. They areoften produced as 50 to 80 weight % UAN solution. The strength of UANsolutions is sometimes expressed as weight percent of elementalnitrogen. A 65% UAN solution by weight UAN is comparable to a 32%elemental nitrogen weight percent solution. Lower strength solutions canbe prepared, and may often be prepared by the agricultural user, bydiluting industrially produced UAN solutions. UAN solutions higher than80 weight % UAN may experience precipitation problems. Additionally,corrosion risks generally increase with the stronger UAN solutions.

[0011] Corrosion risks are associated with the both the production,storage, transportation, and use of UAN. Production piping, tanks, andother equipment, usually comprised of ferrous metals, are subjected tolong term exposures and corrosive forces during the production of UAN.Agricultural users of UAN solutions often have ferrous equipment andtanks that are subject to the same corrosive forces. Transporters of UANsolutions often use ferrous equipment, such as storage tanks, tankertrucks, rail cars, and/or barges to store and transport UAN solutions.Both producers, transporters, and users of UAN solutions benefit fromcorrosion inhibitors added to the UAN solution during production or postproduction.

[0012] The present invention comprises a combination of two or morecorrosion inhibiting compounds blended and added to the UAN solution.One of the compounds is a form of molybdate. Typically an aqueoussolution of alkali metal molybdate would be utilized, such as Sodium orPotassium molybdate (Na2MoO4 or K2MoO4), however, any other form ofmolybdate that can deliver a molybdate ion can be used. The otheradditive or additives, referred to herein as secondary inhibitors, wouldbe one or more of the inorganic phosphates (including phosphates,polyphosphates, and pyrophosphates) and organic phosphates orphosphonates. Inorganic phosphates include, but are not limited to, SHMP(Sodium Hexametaphosphate) and TKPP (Tetra-Potassium Pyrophosphate).There are numerous other inorganic phosphates that will also serve assuitable secondary inhibitors. Organic phosphates or phosphonatesinclude, but are not limited to, HEDP(1-Hydroxyethylidine-1,1-diphosphonic acid; also known as ethanoldiphosphonate, acetodiphosphonic acid, or etidronic acid), ATMP or AMP(aminotri(methylenephosphonic acid)), PBTC (Phosphonobutanetricarboxylic acid), DETPMP (Diethylenetriaminepenta(methylenephosphonic acid)), and HPA (hydroxyphosphono acetic acid). There arenumerous other organic phosphates and phosphonates that will also serveas suitable secondary inhibitors.

[0013] The combination of molybdate with one or more of the phosphateshas surprisingly resulted in lower corrosion rates than either compoundused alone, even at comparable dosage rates, and has resulted in cleanerUAN solutions with little iron or other material precipitates than witheither inhibitor used alone. Precipitates are undesirable, both from aproduct quality standpoint, but also because precipitates can causelocalized corrosion, by creating a site which the inhibitor cannotreach, sometimes referred to as under-deposit corrosion. The avoidanceof precipitates by the use of phosphorous containing compounds preventsthe occurrence of under-deposit corrosion. The molybdate/phosphatecombination also reduces or eliminates the pitting of ferrous metals,often seen in UAN solution exposed metals protected by the use offilmer-type corrosion inhibitors. The limited corrosion experienced withthe molybdate/phosphate inhibitor appears to be evenly distributed overthe surface of the exposed metal.

[0014] The dosage of molybdate and phosphate inhibitors can vary withina wide range, limited on the low side by the amount needed to insureadequate corrosion inhibition (taking into account that UAN solution maybe further diluted by the agricultural user prior to use), and on thehigh side by the cost of the inhibitors and by the solubility of theinhibitors in the UAN solution. Amounts of molybdate (as MoO4) can rangefrom 1 ppm to 500 ppm by weight of UAN solution, with the preferredrange from 10 ppm to 200 ppm. Amounts of phosphate can also range fromlevels as low as 1 ppm up to 500 ppm, with the preferred range beingfrom 5 ppm to 50 ppm.

[0015] The inhibitor combination may be added to the UAN solution in avariety of ways. The inhibitors typically would be combined, and thenadded. However, that may also be added to the UAN independently. Inaddition, the inhibitors may be added during the production process ofthe UAN solution, or after production is complete. However, lateraddition of the inhibitors to the UAN solution obviously limits theprotection afforded to the process equipment.

EXAMPLES

[0016] Tests were run on carbon steel coupons using a variety ofphosphate compounds combined with molybdate. Table 1 summarizes theresults of the first of these tests. An uninhibited 65 weight % aqueoussolution of UAN was utilized (32% elemental Nitrogen). UAN solution wasadded to a 1 liter volumetric flask, and the volumes of requiredinhibitor compounds required to equal the proper dosage, as set forth inTable 1, were calculated. Each of the individual specific gravities ofeach solution was determined at 60 degrees F. using a 10 ml precisionpicometer. The individual test solutions were prepared by withdrawingaliquots of each corrosion inhibitor using a one CC precision syringeand injecting the resulting volume into the one liter volumetric flaskcontaining the UAN solution. A water bath maintained at 89 degrees F.was utilized. Beakers containing the test solutions and metal testcoupons were placed into the water bath. The ppm dosages of eachinhibitor is expressed in relation to the weight of the UAN solution,including water.

[0017] Mild carbon steel coupons (alloy C1010) were prepared for use.Each coupon was weighed on an analytical balance, then immersed in a 10%HCL solution for one minute, to “activate” the coupons, rinsed with 95%reagent grade ethanol to a pH of 7.0. The coupons were then suspended inthe test solution using a nylon filament.

[0018] Coupons were left in the UAN solutions either 4 days, or 13 days,as shown in Table 1. Upon removal from the UAN solutions, the couponswere gently cleaned using a brush and soap, rinsed with deionized water,dried and weighed. The mils per year loss rate was then calculated basedupon the weight loss in the coupon. The UAN solution was monitoredduring the course of the tests for any foaming, color change, andsludge/precipitate formation.

[0019] Table 1 includes the test results for the following: TABLE I 4Day % 13 Day % 4 Day 13 Day Inh. Inh. mpy mpy 25 ppm MoO₄ 58% 28% 32 5525 ppm MoO₄ 10 ppm SHMP 72% 45% 21 42 25 ppm MoO₄ 10 ppm TKPP 75% 46% 1941 25 ppm MoO₄ 10 ppm HEDP 68% 53% 24 36 25 ppm MoO₄ 30 ppm HEDP 64% 62%27 29 Blank — — 76 76

[0020] Additional tests were run on carbon steel coupons using a HEDP asthe phosphate inhibitor combined with molybdate. Two tests were runusing HEDP alone. The same procedures as those outlined above wereutilized. Table 2 summarizes the results of these tests. Table 2includes the test results for the following: TABLE 2 4 Day % 13 Day % 4Day 13 Day Inh. Inh. mpy mpy 25 ppm MoO₄ 37% 17% 36 65 10 ppm HEDP  4% 5% 55 74 30 ppm HEDP 16% 35% 48 51 25 ppm MoO₄ 10 ppm HEDP 74% 71% 1523 25 ppm MoO₄ 30 ppm HEDP 77% 82% 13 14 15 ppm MoO₄ 30 ppm HEDP 68% 73%18 21 15 ppm MoO₄ 10 ppm HEDP 54% 32% 26 53 Blank — — 57 78

[0021] As evidenced from the attached tables, the combination ofmolybdate and HEDP, SHMP or TKPP showed much better corrosion inhibitionresults than molybdate alone, or the secondary inhibitor HEDP alone.

[0022] Additional tests were run on carbon steel coupons using molybdatewith combinations of phosphate inhibitors. A similar test method as thatdescribed above was utilized. Tests were run on carbon steel coupons(alloy C1018) using a variety of phosphate compounds combined withmolybdate. Table 3 summarizes the results of these tests. The water bathwas maintained at 90 degrees F. and coupons were exposed for 6 and 14days. A 65 weight % uninhibited aqueous solution of UAN was prepared(32% elemental Nitrogen), having a specific gravity of approximately1.343. 500 mls of UAN solution were added to each beaker and the volumeof required inhibitor compounds required to equal the proper dosage, asset forth in Table 3, were calculated. The amount of each compound wasadded to the 500 mls of UAN solution using a syringe, and was mixedusing a teflon stirring rod.

[0023] Mild carbon steel coupons (alloy C1018) were prepared for use.Each coupon was weighed on an analytical balance, carefully degreasedusing methanol, rinsed with demineralized water, and allowed to dry. Thecoupons were then immersed in a 10% HCL solution for one minute, to“activate” the coupons, rinsed with demineralized water, and thenimmersed in the appropriate beaker of UAN solution and inhibitorcompound.

[0024] Coupons were left in the UAN solutions either 6 days, or 14 days,as shown in Table 3. Upon removal from the UAN solutions, the couponswere gently cleaned using a brush and soap, rinsed with demineralizedwater, dried and weighed. The mils per year loss rate was thencalculated based upon the weight loss in the coupon. Table 3 summarizesthe results of these tests and includes the following: TABLE 3

Blanks 25 ppm MoO4, with 10 ppm of HEDP 25 ppm MoO4, with 10 ppm of ablend of 70% HEDP/30% AMP (Aminotri (methylenephosphic acid)) 25 ppmMoO4, with 10 ppm of a blend of 35% HEDP/15% AMP/50% HPA(Hydroxyphosphone acetic acid) PPM Net Sample Phosphate PPM Mo # DaysCoupon Coupon Coupon Weight mpy % Description Dosage Dosage ScreenedNumber Initial Wt. Final Wt. Loss Corrosion Inihibition Blank 0 0 6 249511.3233 10.8296 0.4937 88.6  0% Blank 0 0 14 2496 11.3415 10.3720 0.969574.6 16% HEDP 10 25 MoO₄ 6 2506 11.3035 11.0509 0.2526 45.4 49% HEDP 1025 MoO₄ 14 2507 11.3582 10.5792 0.7790 59.9 32% HEDP 10 25 MoO₄ 14 250811.3640 10.5350 0.8290 63.8 28% 70/30 10 25 MoO₄ 6 2509 11.0735 10.88880.1847 33.2 63% HEDP/AMP 70/30 10 25 MoO₄ 14 2510 11.3450 10.6076 0.737456.7 36% HEDP/AMP 70/30 10 25 MoO₄ 14 2511 11.3850 10.6744 0.7106 54.738% HEDP/AMP 35/15/50 10 25 MoO₄ 6 2512 11.3845 11.2083 0.1762 31.6 64%HEDP/AMP/ HPA 35/15/50 10 25 MoO₄ 14 2513 11.2863 10.5237 0.7626 58.734% HEDP/AMP/ HPA 35/15/50 10 25 MoO₄ 14 2514 11.3805 10.5716 0.808962.2 30% HEDP/AMP/ HPA Acidified Coupon 2515 11.0607 11.0596 0.0011

[0025] The results are expressed as mils per year (mpy), measured bothafter 6 days and 14 days exposure to UAN, and as a percentage ofimprovement over the blank results. Table 3 reveals that themolybdate/phosphate inhibitor blends significantly inhibited thecorrosion of the carbon steel coupons.

[0026] It is anticipated that the use of other pyrophosphates andhexaphosphates would yield similar results. Such compounds wouldinclude, but not be limited to, tetrasodiumpyrophosphate (TSPP), andsodiumtripolyphosphates (STPP).

[0027] The foregoing description of the invention is intended todisclose the best mode for practicing the invention. However, variationson this mode will be apparent to those skilled in the art and areintended to be within the scope of the claims set forth below.

We claim:
 1. A corrosion inhibiting solution for use with Urea AmmoniumNitrate solutions comprising; between 1 ppm and 500 ppm molybdate byweight of Urea Ammonium Nitrate solution, and between 1 ppm and 500 ppmof a phosphate by weight of Urea Ammonium Nitrate solution.
 1. Acorrosion inhibiting solution for use with Urea Ammonium Nitratesolutions comprising; between 1 ppm and 500 ppm molybdate by weight ofUrea Ammonium Nitrate solution, and between 1 ppm and 500 ppm of aphosphate by weight of Urea Ammonium Nitrate solution.
 2. A corrosioninhibiting solution as in claim 1 wherein said phosphate is an inorganicphosphate.
 3. A corrosion inhibiting solution as in claim 2 wherein saidinorganic phosphate is at least one of the compounds selected from thegroup consisting of Sodium Hexametaphosphate and Tetra-PotassiumPyrophosphate.
 4. A corrosion inhibiting solution as in claim 1 whereinsaid phosphate is an organic phosphate.
 5. A corrosion inhibitingsolution as in claim 4 wherein said organic phosphate is at least one ofthe compounds selected from the group consisting of HEDP(1-Hydroxyethylidine-1,1-diphosphonic acid; also known as ethanoldiphosphonate, acetodiphosphonic acid, or etidronic acid), ATMP or AMP(aminotri(methylenephosphonic acid)), PBTC (Phosphonobutanetricarboxylic acid), DETPMP (Diethylenetriaminepenta(methylenephosphonic acid)), and HPA (hydroxyphosphono acetic acid).
 6. Acorrosion inhibiting solution as in claim 1 wherein said phosphate is amixture of inorganic and organic phosphates.
 7. A corrosion inhibitingsolution as in claim 1 wherein said molybdate is in the form of analkali metal molybdate.
 8. A corrosion inhibiting solution as in claim 1wherein said molybdate is in the form of sodium molybdate.
 9. Acorrosion inhibiting solution for use with Urea Ammonium Nitratesolutions comprising; between 10 ppm and 200 ppm molybdate by weight ofUrea Ammonium Nitrate solution, and between 5 ppm and 50 ppm of aphosphate by weight of Urea Ammonium Nitrate solution.
 10. A corrosioninhibiting solution as in claim 9 wherein said phosphate is at least oneof the compounds selected from the group consisting of SodiumHexametaphosphate, Tetra-Potassium Pyrophosphate, HEDP(1-Hydroxyethylidine-1,1-diphosphonic acid; also known as ethanoldiphosphonate, acetodiphosphonic acid, or etidronic acid), ATMP or AMP(aminotri(methylenephosphonic acid)), PBTC (Phosphonobutanetricarboxylic acid), DETPMP (Diethylenetriaminepenta(methylenephosphonic acid)), and HPA (hydroxyphosphono acetic acid).
 11. Acorrosion inhibiting solution as in claim 10 wherein said molybdate isin the form of an alkali metal molybdate.
 12. A corrosion inhibitingsolution as in claim 10 wherein said molybdate is in the form of sodiummolybdate.
 13. A method for inhibiting corrosion in ferrous metalssubjected to solutions of Urea Ammonium Nitrate, including the step ofadding to the solution of Urea Ammonium Nitrate during the production ofthe solution of Urea Ammonium Nitrate, between 1 ppm and 500 ppmmolybdate by weight of Urea Ammonium Nitrate solution and between 1 ppmand 500 ppm of a phosphate by weight of Urea Ammonium Nitrate solution.14. A method as in claim 13, wherein said phosphate is at least one ofthe compounds selected from the group consisting of SodiumHexametaphosphate, Tetra-Potassium Pyrophosphate, HEDP(1-Hydroxyethylidine-1,1-diphosphonic acid; also known as ethanoldiphosphonate, acetodiphosphonic acid, or etidronic acid), ATMP or AMP(aminotri(methylenephosphonic acid)), PBTC (Phosphonobutanetricarboxylic acid), DETPMP (Diethylenetriaminepenta (methylenephosphonic acid)), and HPA (hydroxyphosphono acetic acid).
 15. A methodas in claim 13, wherein said molybdate is in the form of an alkali metalmolybdate.